Field
Various communication systems may benefit from buffer reporting. For example, in systems in which user equipment may use dual connectivity, it may be useful to provide cross-reporting of empty or non-empty buffers to each base station.
Description of the Related Art
In dual connectivity (DC), a UE is able to be simultaneously connected to both a Master evolved node B (MeNB) and a Secondary eNB (SeNB) with two medium access control (MAC) entities configured for the master cell group (MCG) and secondary cell group (SCG) corresponding to the MeNB and SeNB respectively. Buffer status reports (BSR) can be used to report uplink (UL) buffer status of the UE to the eNB.
With two different MAC entities in dual connectivity, BSR to the MeNB can include buffer status of the bearers for which a logical channel is used in the MAC entity of the MCG, namely the MCG bearers and the split bearer(s). Similarly, BSR to the SeNB can include buffer status of the bearers for which a logical channel is used in the MAC entity of the SCG, namely the SCG bearers and the split bearer(s).
With an independent scheduler located at two eNBs connected with a non-ideal backhaul (X2 in DC), MeNB and SeNB may not be aware of the scheduling decision of the other scheduler. Power control, therefore, may become challenging. For example, combined scheduling could exceed UE maximum power in a case that both eNBs allocate too many physical resource blocks (PRBs).
A study on small cell enhancements was conducted in 3GPP, and is reported at 3GPP RP-122033 (see also RP-132069 and R2-140906), each of which three reports are hereby incorporated herein by reference in its respective entirety. In order to decrease signaling load toward the core network, as well as benefiting from flexible resource usage across eNBs, dual connectivity was investigated. In DC as studied, a UE is simultaneously connected to both a Master eNB (MeNB) and a Secondary eNB (SeNB). MeNB and SeNB are assumed to be connected via X2. The main characteristic of X2 in DC as studied is that it is a non-ideal backhaul link: transmission delays in the range of ˜20 ms can happen, and the bit rate is limited, as described at 3GPP TS 36.932, which is hereby incorporated herein by reference in its entirety. The outcome of the study can be found in 3GPP TR 36.842, which is hereby incorporated herein by reference in its entirety. The conclusion of that report was that two different user plane architectures may be supported: 1A and 3C—shown in FIG. 1.
The cells from MeNB can be defined as Master Cell Group (MCG), and the cells from SeNB can be defined as the Secondary Cell Group (SCG). Two MAC entities can be configured to the UE for the MCG and SCG respectively.
FIG. 2 illustrates different type of bearers. Architecture 1A and 3C can be realized by different RRC configuration which leads to three different types of bearers: those bearers 210 served by MeNB alone, namely MCG bearers; bearers 220 served by MeNB and SeNB, also known as split bearer; and bearers 230 served by SeNB alone, namely SCG bearers.
For split bearer 220, two radio link control (RLC) entities can be used: one RLC entity for the MeNB and one RLC entity for the SeNB. Once a packet data convergence protocol (PDCP) packet data unit (PDU) is delivered to one RLC entity, all possible retransmissions may be managed by that RLC entity. Also, RLC status reports can conventionally only be exchanged by peer RLC entities of the same eNB. That is to say that RLC status reports for the MeNB can conventionally only be exchanged between the UE and the MeNB, while RLC status reports for the SeNB can conventionally only be exchanged between the UE and the SeNB.
Buffer status reports (BSR) can be used to report uplink (UL) buffer status of the UE to the eNB. Different logical channels (LCH) can be configured to different logical channel groups (LCGs) and the buffer status value can reflect data available for transmission for each LCG, as described at 3GPP TS 36.321 and 36.331, which are both hereby incorporated herein by reference in their respective entireties.
With two different MAC entities in dual connectivity (DC), a BSR to the MeNB can include buffer status of the bearers for which a logic channel is used in the MAC entity of the MCG, namely MCG bearers, and the split bearers, at least for the data available for transmission in the RLC layer of MCG. Similarly the BSR to the SeNB can includes buffer status of the bearers for which a logic channel is used in the MAC entity of the SCG, namely SCG bearers, and split bearers, at least data available for transmission in RLC entity of SCG. How the data available for transmission in PDCP layer of split bearer is still under discussion in 3GPP.
As noted above, with an independent scheduler located at two eNBs connected with a non-ideal backhaul, MeNB and SeNB may not be aware of the scheduling decision of the other scheduler. Power control, therefore, is challenging as UE maximum power could be exceeded if both eNBs allocate too many PRBs. Conversely, UE power could be underutilized if both schedulers are too conservative, which could reduce UL throughput.
One approach is for the UE in dual connectivity to always send two BSRs, one reflecting the status of each MAC entity. This approach may require each eNB to be aware of the BSR configuration of the other. Furthermore, this approach may use significant overhead.